Procedure for producing a photoconductive device



March 31, 1959 A. L. FLOYD, JR

PROCEDURE FOR PRODUCING A PHOTOCONDUCTIVE DEVICE Filed Oct. 1. 1956 GLASS 545E.

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PROCEDURE FOR PRODUCING A PHOTO- CONDUCTFJE DEVICE Acey L. Floyd, Jr., Duarte, Calif., assignor, by mesne assignments, to Dresser Industries, Inc., Dallas, Tex., a corporation of Delaware Application October 1, 1956, Serial No. 612,929

7 Claims. (Cl. 117-201) This invention relates to photoconductive devices using the known photoconductivity properties of cadmium sulphide; and more particularly the invention relates to a device having a novel photoconductive film or deposit, and a method or procedure for producing the device. Still more specifically, the invention relates to devices comprising a thin polycrystalline film or deposit composed principally of cadmium sulphide, which film or deposit is made highly photoconductive; and a method of so forming the film that it is, in final form, possessed of linear voltage versus current characteristics and of high dark resistivity and high photoconductive sensitivity characteristics.

It is known in the prior art that cadmium sulphide powder as commercially produced has photoconductive properties. The photoconductivity thereof apparently is due to one or more impurities in the powder, and is very variable or non-uniform with respect to different lots of the material. Further, the sensitivity is relatively low as compared to the photoconductivity sensitivity of a single cadmium sulphide crystal. However, single crystals are not only relatively expensive but quite unsuitable for many applications, especially where large photoconductive areas are necessary; and accordingly the finely divided or powder form of cadmium sulphide has been used in the preparation of photoconductive films or areas. See, for example, Journal of the Optical Society of America 45, 647 (1955), disclosing apparently a superior grade of powder CdS photosensitive cell produced by Nicol-Kazon. Those cells were, however, as were all previous CdS cells known to applicant, possessed of voltage-versus-current characteristics far from linear; giving currents varying as the fourth or fifth power of the voltage. It is believed that this non-linearity was due to barriers formed between the powder particles, since sintered cells may have a linear characteristic (see Thomsen and'Bube, Review of Scientific Instruments, 26, 664; 1955).

The present invention provides a powder-type film or deposit of material which has high dark resistance, exceptionally high photoconductive sensitivity, and linear voltage-versus-current photoconductivity characteristics; and provides a method or procedure for consistently producing such films or deposits, or cells comprising such films or deposits, with a remarkable degree of uniformity. Further, photoconductive cells produced in accord with the novel procedure of the invention are capable of retaining their desirable photoconductive sensitivity characteristics with an exceptional degree of stability over an extended period of time. It has been experienced that commercially available photoconductive cells have sensitivities which depreciate quite rapidly with age.

In view of the preceding stated considerations and objectionable features of the prior art, it is an object of the present invention to provide a simple procedure for preparing a photoconductive device having a photoconductive surface or film of high photosensitivity and long work life.

Another object of the invention is to provide a photoconductive device of improved electro-optical characteristics. An additional object is to provide a photoconductive cell of the thin-film type having photoconductive sensitivity substantially equal to that of the powder employed in producing the film.

Another object is to provide an improved photoconductive layer or film-like deposit.

Another object is to provide an improved cadmiumsulphide photoconductive cell.

Other objects and advantages which Will hereinafter be made apparent are attained by the invention'which in the article form is illustrated in the accompanying drawing.

The generally-practiced procedure in producing thinfilm type photoconductive cadmium sulphide cells has consisted in part in sublimating in a vacuum chamber a photoconductive cadmium-sulphide powder, sometimes with an added material as a so-called impurity, the vaporized or Sublimated cadmium sulphide being at least in part received as a film-like thin deposit on a suitable cell base such as, for example, glass. Subsequent heattreatment of cells thus produced provided an increased degree of photoconductive sensitivity, but the heating was very critical, the sensitivity increasing at a very high rate to a maximum value as heating progressed and then rapidly decreasing to a very low value at which the cell was, for practical purposes, useless and after which time further heat-treatment was of no avail in restoring a satisfactory degree of photoconductive sensitivity. Further, the heat-treatment required for maximum sensitivity for different cells and for different batches of cells was widely variant; and cells produced were far from uniform in characteristics.

The present invention provides a procedure whereby such thin-film cadmium sulphide cells are invariably brought to a maximum degree of sensitivity, with a very high dark resistance and a very high dark resistance versus light resistance ratio. By this novel procedure the heat-treatment of the cell is rendered non-critical, and uniformly excellent photoconductive cells may readily be produced. It has been determined that the deterioration in photoconductive sensitivity of a CdS thin-film deposit following attainment of maximum sensitivity, as heating is continued, is apparently caused by creation of a thin surface layer of material which is not light-sensitive or is in effect a poison on the cell, and that this thin surface layer prevents the underlying particles or crystals of cadmium sulphide material from collectively responding to illumination. Further, it has been discovered that the thin layer of non-sensitive material which thus poisons the underlying material may readily be removed, as by a dilute acid wash and water rinse, and that when thus rid of the surface layer of inactive or poisonous material, the cell regains and retains its previously-destroyed maximum photoconductive sensitivity. A suitable acid wash and rinse is provided by dipping a heat-treated cell in a dilute sulphuric acid solution followed by rinsing with distilled water.

As an example, atypical cell, made according to the previously-outlined vacuum-sublimation procedure and heated in air until its photoconductive sensitivity attained a maximum and then decreased to substantially no sensitivity to light, was then cleaned by dipping in dilute sulphuric acid, rinsed with distilled water, and dried. The cell then had a dark surface resistance of 2000 megohms, and under ordinary room illumination a resistance of 0.2 megohm, thus representing a dark-versuslight resistance ratio of 2000/0.2, or in excess of 9000. It was found that cells could easily be brought to maximum sensitivity by the described procedure even after more than an hour of excess heating subsequent to deaseou. 1 9

crease of; photoconductive:sensitivity to near-zero value.

As a. specific example illustrating procedure according to the invention, the following is set forth in detail:

An electrically-insulative base in the form of a glass slide was thoroughly cleaned and dried. A geometrical array of discrete films of indium was formed on the base, the films being separated by line-like areas free of indium. The procedure employed was substantially the same as that disclosed in applicants copending application Serial No. 489,397, filed February 21, 1955, employing, however, indium rather than chromium as a conductive material. A mixture of commercial grade cadmium sulphide in fine powder form, with approximately 10,000 parts of cupric chloride per 1,000,000 parts of cadmium sulphide (based on metal component) was prepared. The powder mixture was sublimated in vacuo and part of the sublimation product there received as a substantially uniform film or deposit over and between the conductive films on the slide. After release of the vacuum the slide and material thereon were heated in vacuum at about 450 C. for a period in excess of live minutes. The slide was allowed to cool and was then washed, with gentle swabbing with cotton, in dilute H 80 for about fifteen minutes, and then rinsed and dried. The sensitivity was found to be equal to that of the powder prior to sublimation.

It is generally immaterial whether electrodes are applied to the cell base prior to deposition of the sublimation product, or subsequently. In either event, electrodes preferably are of a, material providing electrical contact with the photoconductive film or deposit, and the electrode material may be such as is disclosed in concurrently filed patent application of Czipott et al., Serial No. 613,172.

As may be inferred from the preceding description, the necessary heat-treatment is not critical, but can be carried to reasonable extremes without danger of producing a defective cell.

From the preceding it is evident that modifications and variations of the procedure and means according to the invention will occur to those skilled in the art; and accordingly it is not desired to be limited to thespecific details of the exemplary procedure and means hereinbefore described, but what is claimed is:

1. A procedure for producing a high-sensitivity photoconductive deposit, comprising: preparing a cell base; depositing on the base in vacuo a thin film-like deposit of a photoconductive material comprising principally cadmium sulphide; heat-treating said deposit of material to an extent exceeding that at which the deposit attains maximum photoconductive sensitivity; and thereafter subjecting the thus-treated deposit to a dilute acid wash to restore photoconductive sensitivity to said deposit.

2. A procedure for preparing a cadmium sulphide photoconductive thin film of high sensitivity, comprising: preparing a clean base surface on which to form the thin film; preparing a finely divided photosensitive material comprising at least a major proportion of cadmium sulphide; sublimating the material in vacuum and there concurrently receiving on the base surface at least a portion of the sublimation product in a thin film-like deposit; heating the base surface and deposited material to an extent greater than is required for attaining maximum photoconductive sensitivity of the deposited material; and washing at least the exposed face of the deposited material with dilute sulphuric acid to restore photoconductive sensitivity thereto.

3. A procedure for producing a high-sensitivity photoconductive cadmium sulphide film, comprising, in sequence: sublimating cadmium sulphide in vacuum and there receiving a substantially continuous thin film-like deposit of the product of the sublimation on a suitable base; in said vacuum, heating the deposit to render the deposit photoconductively sensitive, and continuing the heating beyond the point of attainment of maximum photoconductive sensitivity thereof to a point of substantially reduced photoconductive sensitivity; and subsequently restoring photoconductive sensitivity to said depositby washing the surface thereof with dilute sulphuric acid.

4. A procedure for producing a high-sensitivity photoconductive cell, comprising: subliming in vacuo a material composed principally of finely divided cadmium sulphide and there receiving on a cell base a thin deposit of the sublimation product; heating the base and deposit of sublimation product beyond the point of attainment of optimum photoconductive sensitivity of the deposit; removing a surficial portion of the deposit witha dilute acid wash; rinsing the cell base and remaining portion of the thin layer with water; and drying the thus produced cell.

5. A procedure for producing a highly sensitive photoconductive device, comprising forming a thin film-like deposit of a photoconductor material, heat-treating the deposit beyond the stage of attainment of maximum photoconductive sensitivity during heat-treatment, and removing a surficial portion of the deposit by dilute acid treatment to restore photoconductive sensitivity to said deposit.

6. A photoconductive device comprising a base, and a thin, film-like deposit of a polycrystalline photoconductor on said base, said deposit having an acid-etched highly photoconductively sensitive surface.

7. A photoconductive device comprising aninsulating base member; a thin vapor-deposited layer of photosensitive cadmium sulphide on said base member which layer has been treated by washing in a dilute solution'of sulphuric acid.

References Cited in the file of this patent UNITED STATES PATENTS 2,116,977 Laise May 10, 1938 2,675,331 Cusano et a1. Apr. 13, 1954 2,691,601 Butler et al Oct. 12, 1954 FOREIGN PATENTS 157,179 Australia June 23, 1954 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,880,119 March 31, 1959 Acey L. Floyd, Jr.

It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 34, for "photoconductivity" read photoconductive column 3, line 25, for "H 80" read H 80 Signed and sealed this 18th day of August 1959.

(SEAL) Attest:

KARL H, AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 2,880,119 March 31, 1959 Acey L. Floyd, Jr.

It is hereby certified that error appears in theprinted specification of the above "numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 34, for 'photoconchlctivity" read photoconductive column 3 line 25, for H SO" read v H 50 (SEAL) Attest:

KARL Ha AXLINE Attesting ()flicer ROBERT C. WATSON Commissioner of Patents 

1. A PROCEDURE FOR PRODUCING A HIGH-SENSITIVITY PHOTOCONDUCTIVE DEPOSIT, COMPRISING: PREPARING A CELL BASE, DEPOSITING ON THE BASE IN VACUO A THIN FILM-LIKE DEPOSIT OF A PHOTOCONDUCTIVE MATERIAL COMPRISING PRINCIPALLY CADMIUM SULPHIDE, HEAT-TREATING SAID DEPOSIT OF MATERIAL TO AN EXTENT EXCEEDING THAT AT WHICH THE DEPOSIT ATTAINS MAXIMUM PHOTOCONDUCTIVE SENSITIVITY AND THEREAFTER SUBJECTING THE THUS-TREATED DEPOSIT TO A DILUTE ACID WASH TO RESTORE PHOTOCONDUCTIVE SENSITIVITY TO SAID DEPOSIT. 